7-substituted quinone methides as inhibitors for unsaturated monomers

A compound of formula I ##STR5## wherein R.sub.1 and R.sub.2 are independently alkyl of 4 to 18 carbon atoms, cycloalkyl of 5 to 12 carbon atoms or phenylalkyl of 7 to 15 carbon atoms, and

• R.sub.3 is --CN, --COOH, --COOR.sub.4, --COR.sub.5, --OCOR.sub.6, --CONR.sub.7 R.sub.8 or --PO(OR.sub.9).sub.2 where
• R.sub.4 is alkyl of 1 to 18 carbon atoms, cycloalkyl of 5 to 12 carbon atoms, phenyl or benzyl,
• R.sub.5 is alkyl of 1 to 18 carbon atoms, aryl of 6 to 10 carbon atoms or said aryl substituted by 1 or 2 alkyl of 1 to 4 carbon atoms or by hydroxyl,
• R.sub.6 is alkyl of 1 to 18 carbon atoms, aryl of 6 to 10 carbon atoms or said aryl substituted by 1 or 2 alkyl of 1 to 4 carbon atoms or by hydroxyl,
• R.sub.7 and R.sub.8 are independently hydrogen, alkyl of 1 to 18 carbon atoms or said alkyl substituted by alkylamino of 1 to 4 carbon atoms, by dialkylamino of 2 to 8 carbon atoms or by hydroxyl;
• benzyl, aryl of 6 to 10 carbon atoms or said aryl substituted by alkyl of 1 to 4 carbon atoms, by alkylamino of 1 to 4 carbon atoms, by dialkylamino of 2 to 8 carbon atoms, by phenylamino or by hydroxyl, or
• --NR.sub.7 R.sub.8 is morpholino, piperidino or pyrrolidino, and
• R.sub.9 is hydrogen or alkyl of 1 to 18 carbon atoms, but excluding
• a. (3,5-di-tert-butyl-4-oxocyclohexa-2,5-dienylidene)acetonitrile;
• b. (3,5-di-tert-butyl-4-oxocyclohexa-2,5-dienylidene)acetic acid;
• c. methyl (3,5-di-tert-butyl-4-oxocyclohexa-2,5-dienylidene)acetate;
• d. diethyl (3,5-di-tert-butyl-4-oxocyclohexa-2,5-dienylidene)methanephosphonate;
• e. (3,5-di-tert-butyl-4-oxocyclohexa-2,5-dienylidene)methyl acetate;
• f. (3,5-di-tert-butyl-4-oxocyclohexa-2,5-dienylidene)methyl pivalate;
• g. (3,5-di-tert-butyl-4-oxocyclohexa-2,5-dienylidene)methyl benzoate;
• h. 2,6-di-tert-butyl-4-(2-oxophenylethylidene)-cyclohexa-2,5-dienone;
• and
• i. diisopropyl (3,5-di-tert-butyl-4-oxocyclohexa-2,5-dienylidene)methane-phosphonate.